Evolutionary origin of the <i>MAT</i> locus in <i>Sclerotinia sclerotiorum</i> Inv- isolates from hypothetical ancestors by means of a double crossing over, an inversion and a single crossing over.

<p>Genes are boxes, directions of transcription are indicated by arrows, gene names are inside the boxes, positions of the 250-bp motif are indicated by red horizontal lines, crossing overs are marked by ‘X’, the inversion by a curved arrow. 4A) A double crossing over between ancestral <i>MAT1-1</i> (blue) and <i>MAT1-2</i> (green) transferred a 250-bp fragment, the ‘250-bp motif’, from <i>MAT1-1-1</i> to <i>MAT1-2-1</i> and flanking region. 4B) A subsequent inversion (gray) in the ancestral <i>MAT1-2</i> (green), followed by a crossing over, 4C) with an ancestral <i>MAT1-1</i> (blue), results in the <i>MAT</i> arrangement in <i>S. sclerotiorum</i> Inv- isolates. The asterisk indicates the location of the DNA sequence alignment provided in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0056895#pone-0056895-g005" target="_blank">Figure 5</a>.